Metallic vapor arc-lamp having high intensity sun-like emission



United States Patent 3,521,108 METALLIC VAPOR ARC-LAMP HAVING HIGHINTENSITY SUN -LIKE EMISSION Rodney E. Hanneman, Burnt Hills, N.Y.,assignor to General Electric Company, a corporation of New York FiledJuly 17, 1968, Ser. No. 745,502

Int. Cl. H01j 17/20, 61/22 U.S. Cl. 313-184 14 Claims ABSTRACT OF THEDISCLOSURE The present invention relates to metal vapor arc-lampsutilizing a plurality of metallic vapors as the dischargesupporting andlight-emitting media and which produce light of a spectral distributionso as to achieve substantially sun-like or white emission at extremelyhigh efficacies.

In the past, the standard arc-lamp light sources for the production ofhigh-intensity light were the low pressure sodium lamp and the mercuryvapor lamp. The low pres sure sodium vapor lamp had a distinctdisadvantage, in that its light emission was substantially that of thesodium D-line, a couplet having wavelengths of 5,896 and 5,890 A.U.,respectively. This emission is chromatically' unbalanced and generallynot acceptable.

Emission of the prior art mercury vapor lamps likewise suffered fromchromatic deficiencies, in that the emission thereof had a substantiallybluish tint under which reds did not appear in their true color, thusrendering such light acceptable only for large area street lighting and,even in such instances, such lamps were only competitive withincandescent lamps, and exhibited eflicacies of only about 60 lumens perwatt, at best.

In recent years, significant and substantial advances have been made inthe art to improve metallic vapor arc-lamps, particularly with respectto the efficacy and chromatic response thereof. More specifically, themercury-metallic-halide electric discharge lamp as disclosed and claimedin Pat. 3,234,421-Reiling, issued Feb. 8, 1966, brought to the lightingindustry a lamp which had higher efiicacies than that of the mercuryvapor lamp such that, in commercial operation, efiicacies of 80 to 90lumens per watt are readily available. Additionally, the chromaticcharacteristic of these lamps was such as to approximate closely that ofWhite light. Although the lamps of the general class exemplified by theaboveidentified Reiling patent, and further developments thereof, are asubstantial improvement over mercury vapor lamps, the efficacies arestill somewhat short of the desired and theoretically achievableeflicacies from metallic vapor arc-lamps. Some problems in lumenmaintainance have also been encountered in such lamps.

In yet another and highly significant advance in the lighting art, thehigh pressure sodium vapor arc-lamp, such as is disclosed and claimed inPat. 3,248,590 Schmitt, issued Apr. 26, 1966, brought to the lightingart a new lamp utilizing a light-transmissive, high temperature arc-tubewith a pair of electrodes therein and fillings including light-emittingsodium vapor at greatly increased pressures as compared with the priorart sodium vapor lamps, generally in the order of 30 to 1,000 torr ofsodium. Such lamps, utilizing line-broadening and the increased efiicacyof high-temperature operation, are capable of the attainment ofhigh-lumen efficacies, in excess of lumens per watt with achromaticallyacceptable emission.

Although such lamps represent a very great advance in the lighting artssearch for the ultimate in metallic vapor arc-lamps, the spectraldistribution of the high pressure sodium vapor lamps is still less thandesired in some applications, in that the lamp has a golden spectralemission, weak in green radiation. Although such lamps rendersubstantially improved chromatic response over mercury and low pressuresodium vapor lamps and permit the identification of reds and othersimilar colors, not identifiable under prior art lamps, the emissionthereof is not fully acceptable in some lighting applications in which amore nearly white spectrum is required.

Accordingly, it is an object of the present invention to provide metalvapor arc-lamps having very high lumen efiicacies and substantiallysun-like, White chromatic emission which is completely acceptable forgeneral illumination purposes.

Yet another object of the present invention is to provide metal vaporarc-lamps having a plurality of ionizable metallic species therein whichare compatable with one another to create thermodynamic conditionswithin the arcing volume of a vapor arc-lamp whereby the presence of allspecies are compatible with one another and consistent operation overlong periods of time is achievable.

Still another object of the present invention is to provide metal vaporarc-lamps having a plurality of metal species in the charge addedthereto which charge is partially vaporized to provide a light-emittingmetallic vapor arc which exhibits the characteristics of good chromatic,high-efficacy operation, and long-life operation at a highlumenefiicacy.

Briefly stated, in accord with one embodiment of the present invention,I provide a metallic vapor arc-lamp including an arc-tube composed of ahigh-temperatureresistant, light-transmissive ceramic material, which isalso resistant to chemical attack, containing therein a pair ofoppositely-disposed solid or nonmelting arc-electrodes which definetherebetween an arc path. Within the lamp envelope, I provide a chargecomprising sodium and at least two or more additional atomic speciesfrom the group including cadium, thallium, and mercury. The foregoingconstituents are used under such conditions that the presence of some ofeach of the chosen constituents in a non-volatilized reservoir withinthe lamp is assured at the operating temperatures and further chosen insuch proportions as to provide thermodynamic conditions which insurewhite or near-white spectral emission, together with the characteristicof long maintainance at high efficacy.

The novel features believed characteristic of the present invention areset forth in the appended claims. The invention itself, together withfurther objects and advantages thereof, may best be understood byreference to the attached drawing in which:

FIG. 1 illustrates, in schematic vertical cross section, a typical lampconstructed in accord with the present invention,

FIG. 2' illustrates, in graphic form, the nonlinear activitycharacteristic of a typical constituent binary system of charge metalsuseful in understanding some characteristics of lamps made in accordwith the present invention.

As shown in FIG. 1, high-pressure metallic vapor arclamp 1 comprises anouter, transparent vitreous envelope or jacket 2 of elongated ovoidshape. Neck 3 of envelope 3 2 is closed by a reentrant stem 4, having apress 5 through which extend stilf wire inleads 6 and 7, connected attheir outer ends to threaded shell 8 and center contact 9 of aconventional screw base.

The inner envelope, or arc-tube, 12 which forms the discharge lamp tubeproper is, for example, made of sintercd, high-density, polycrystallinealumina ceramic such as is disclosed and claimed in US. Pat. No.3,026,210 Coble, Transparent Alumina and Method of Preparation."Alternatively, any other similar light-transmissive, high-temperatureceramic which is resistant to chemical reaction with sodium may be used.Tungsten electrodes 16 and 17 are supported in position at the upper andlower ends of tube 12 by dummy end cap 18 and exhaust end cap 19,respectively, hermetically sealed to the arc-tube. The shanks ofelectrodes 16 and 17 are supported from niobium end caps through niobiumtubes 20 and 21, respectively, which project hermetically through theend caps. Each electrode may, for example, comprise a double-woundtungsten wire coil, with interstices filled with electron-emittingmaterial, suitably alkaline earth oxides including barium oxide, forexample. Tube 21 is pierced through at 22 and is used as an exhaust tubeduring manufacture and to introduce an inert gas filling such as xenonand the ionizable dissociable light-emitting charge into the arc-tube.The lower end of tube 21 is thereafter hermetically pinched off by acold weld at 23. A quantity of the charge, somewhat exaggerated for easeof illustration, is shown at 24 in the lower end cap 19. Excess chargemay also collect in the projecting portion of exhaust tube 21, whichtends to be at a cooler temperature in operation of the lamp.

The upper end of arc-tube 12 is supported within en velope 2 by means ofband and rod frame 25 which extends from inlead 6 at the stem end to adimple '27 at the dome end to which it is anchored by a resilient clamp28. The lower end of the arc-tube is connected in inlead 7 by band 29and a short length of rod 30. A strap 31 mechanically interconnects rodsand 30 to stiffen the assembly, while insulator 32 prevents ashort-circuit. The interenvelope space is evacuated in order to conserveheat and this is done prior to sealing off the outer envelope.Thereafter, a conventional getter, suitably barium metal powder pressedinto channeled rings 33, is flashed after sealing in order to assure agood vacuum. Alternatively, a high molecular weight noble 0r inert gasmay be used.

In order to prevent the presence of oxygen within arctube 12 and theconsequent formation of sodium aluminate or similar compounds with thealumina of the arctube 12 or with constituents of the seals, an oxygengetter may be incorporated within the arc-tube or within the inter-bulbspace, as for example as a deposit of a reactive metal getter 43 on tab44 suspended by rod 46 from upper support 25. Such getters are mealshaving, over a given temperature range, a free energy of reaction in theformation of the oxide thereof per mole of oxygen more negative than thefree energy of formation of sodium aluminate per mole of oxygen withinthe alumina arctube. Such getters utilized in high-pressure vaporarclamps including sodium in the vapor state are not my sole invention,but are disclosed and claimed in the copending application of Hanneman,Jorgensen, and Charles entitled High Pressure Sodium Vapor Lamp, Ser.No. 616,538 now abandoned, filed Feb. 2, 1967, of which application Ser.No. 753,143, filed Aug. 16, 1968, is a continuation-in-part and both ofwhich are assigned to the present assignee.

The inert gases which may be used for starting purposes in the arc-tubesof electric discharge lamps in accord with the invention, in increasingorder of atomic weight, are helium, neon, argon, krypton, and xenon andpreferably xenon. Since xenon is expensive, for reasons of economy,krypton or argon may be used alone or adimtted with xenon, but generallyat some sacrifice of efficacy. The specific Xenon pressure for maximumefiicacy in a given lamp depends upon many factors. It has beendetermined, however, that pressures of xenon from 5 to torr areacceptable, however, lamps in accord with the present invention arepreferably constructed using approximately 20 torr OI xenon.

Lamps in accord with the present invention, the structure of which isdescribed hereinbefore, are substantially that as disclosed in theabove-identified Schmitt patent. The inventive concept involved, inaccord with the present invention, relates to changes in chargeconstituents and parameters of operation which provide for theachievement of markedly improved spectral emission characteristics, and,not only a retention of the high efiicacies of the characteristic of thehigh-pressure sodium vapor lamp, but in many instances, an increase inthe efiicacv thereof,

due to additional radiating constituents to the metalliccharge containedwithin the lamp envelope.

Basically, in accord with the present invention, the fill charge of thelamp contains predetermined quantities of sodium and at least two, andpreferably all, of the additional materials of the group includingthallium, cadmium, and mercury.

Prior art as far back as 1916, and the writings and patents of C. P.Steinmetz, for example US. Pat. No. 1,025,932, have recognized that, intheory, it might be possible to correct the spectrally undesirableemission of a given metallic vapor arc-lamp by the addition ofconstituents which supply, when operated alone, a spectral emissioncharacteristic which is lacking in that of the lamp desired to beimproved. Thus, for example, one may readily suggest that thehigh-pressure sodium vapor lamp of the Schmitt patent, being somewhatdeficient in the green area of the visible spectrum at its operatingtemperatures, could be improved by the addition of a green-emittingmaterial, such as thallium to remedy this deficiency. As a practicalmatter, however, one may not simply add an amount of thallium,predetermined from its own vapor pressure and temperaturecharacteristics, to such a lamp and achieve the desired objectives. Suchadditions not only fail to correct the spectral emission, but maydeleteriously affect the lamp efficacy and voltage.

From my work I have determined that the thermodynamics of theliquid-vapor equilibrium conditions existing within a lamp of themetallic vapor arc discharge type contains a number of parameters whichare mutually dependent, and that to change one parameter, greatlyaffects other parameters. Thus, for example, the addition of thallium toa sodium vapor lamp, greatly affects the characteristics of the sodiumalready present within the lamp as contributing to the spectralemission.

Due to the foregoing, and to the number of the variables in such lamps,including the number and quantity of additives, the lamp operatingtemperature, as well as current and voltage therethrough, it is avirtual impossibility to determine the amount and identity of theconstituents of a charge in a metal vapor arc-lamp which will achievewhite light at efiicacies in excess of 100 lumens per watt by trial anderror methods.

In general, I have discovered that the activity of a material, which isdefined as the ratio of a partial pressure within an enclosed space overan alloy, containing the given material, to the partial pressure of thesame material in the absence of any other constituent in equilib riumwith its own liquid or solid phase, may be greatly affected by thepresence of other constituents within the system.

FIG. 2 of the drawing represents a typical activity diagram for a simpletwo-constituent system of sodium and thallium. In FIG. 2, the axis ofabscissas runs from Zero percent thallium and 100 percent sodium, at oneend, to 100 percent thallium and zero percent sodium at the other end.The left axis of ordinates runs from an activity for sodium from 0 to1.0 and right hand axis of ordinates runs from an activity of from 0 to1.0 for thallium, the

activities being normalized to take into account the differing vaporpressures of the pure constituents. In FIG. 2, the diagonal linesrepresent the ideal linear variation which might be expected of theactivities, while the curved lines represent the actual activities.

As may be seen from the graph of FIG. 2, the addition of low percentagesof thallium to a charge originally consisting of sodium, produces a verylow activity of thallium and hence the corresponding vapor pressure ofthallium over the liquid will be much less than over pure thallium andalso substantially less than over an ideal linear type solution.Similarly, at very low thallium concentrations, the concentration and,hence pressure, of the sodium in the vapor phase is not seriouslyaffected. (The same is true for thallium pressures for small additionsof sodium at the high thallium end of the diagram.) Of great interest,however, is the fact that by the time a quantity of thallium has beenadded which is sufficient to cause a significant presence of thallium inthe vapor phase, the amount of sodium in the vapor phase has, by theaddition of thallium, been greatly decreased, so that the activity ofboth the sodium and thallium is very much less than would be present bya similar addition of either one to a lamp in the absence of the other.This foregoing brief description of a simplified two-constituent systemillustrates the type of complexities due to the presence of three ormore simultaneous constituents in the charge of a metal vapor arc-lamp.For this reason, it is obvious that one cannot simply add apredetermined quantity of a given material to the lamp charge, basedupon its own vapor pressure characteristics, to achieve its radiationfrom the lamp without drastically affecting the contribution of theother constituents to such an extent that efficacy and and colorrendition may be seriously and detrimentally affected.

I have further found, quite unexpectedly, that an additional governingcriterion, in the achievement of improved spectral emissioncharacteristics and the maintenance of high-lumen efiicacy in metalvapor arc-lamps utilizing two or more constituents is that, in orderthat the desired proportion, as determined analytically, of a givenconstituent in the vapor phase within the lamp envelop be present, someof that constituent must exist in excess in a liquid phase inequilibrium with the vapor phase. I have further determined that, if oneconstituent of a charge of a plurality of metals in a metal vaporarc-lamp is to be present in excess in the liquid phase within the lampenvelope, then it is required that some of each of the otherconstituents also be present in excess in the liquid phase. When thesecriteria are met, and only then, is it possible to control properly thedesired amount of partial pressure of each of the given constituentswithin the charge of a metallic vapor arc-lamp.

I have further determined that the operating temperature of the metallicvapor arc-lamp must especially take into consideration, and beconsistent with, the vapor pressure characteristics of the lowest vaporpressure metal constituent of the charge. As used herein, the phraseoperating temperature of the lamp is meant to denote the temperature ofthe liquid-metal alloy of the charge which is in equilibrium with thevapor phase within the lamp envelope. In general, this is also thetemperature of the coldest portion of the bulb envelope wall, althoughby extreme, often unnecessary, and unusual seal configurations, it isconceivable that the end seal portions of the,

envelope could be so designed as to cause a thermal gradient through theliquid phase which would cause the coldest portion of the bulb wall tobe somewhat, although not significantly, different from that of theliquid surface in equilibrium with the vapor phase, which is thatportion referred to herein as defining the operating temperature of thelamp. As an example of the foregoing temperature relationship, it is tobe noted that, should thallium be a constituent utilized in the chargeof lamps in accord with the present invention, due to its relatively lowvapor pressure as compared with the other materials utilized herein,operating temperatures must be raised to approximately 750 C. while, onthe other hand, should thallium not be one of the materials utilized, itis possible to operate the lamp at temperatures of less than 700 C.

Another parameter which is of great importance, in the operation of thelamps in accord with the present invention, is the operating temperatureof the lamp. Although a given range of metallic constituents of thecharge added to the lamp envelope may be sufficient to achieve thedesired spectral emission at a particular temperature, such a range ofconstituents is not necessarily correct for the same lamp operated at asubstantially different temperature. Once this is appreciated, however,it does not present an insurmountable problem, because, once the usefulrange of constituents for a given ternary or quaternary system has beendetermined for a given temperature, and the direction of the variationof the quantity of any constituent necessary to produce the desiredspectral output and efiicacy of operation is known, the correct range ofpercentages for operation at different temperatures may be determined.In general, the criteria of the lamps of the invention are as follows:

(1) Irrespective of the number of constituents in the charge, at leastsome of each of the constituents must remain in the unvaporized state atthe lamp operating temperature.

(2) The lamp operating temperature and composition of the added chargemust be chosen so as to cause the desired proportion of each of theconstituents to be in both the liquid and vapor phases individuallywithin the lamp envelope during lamp operation.

(3) The efficacy of the radiation from the lamp under operatingconditions should be of the order of lumens per watt or higher.

(4) The spectral emission of the lamp must have one or more componentswith peak intensities in the range of 5,050 to 5,500 AU, and having acorresponding peak intensity which is at least 10 percent of the peakintensity of the total radiation.

In achieving the foregoing criteria, I cause the charge within the lampenvelope to be chosen from the quaternary system including sodium andtwo or more of thallium, cadmium, and mercury. Although sodium and twoother constituents may be present. I prefer that all constituents bepresent. When all constituents are present, a useful operatingtemperature range of approximately 750 C. to 1,000 C. may be utilized toachieve highly efficacious, substantially-white radiation. In the caseof a quaternary system in which all constituents are present, theoperating range for all temperatures is obtained when the partialpressures of the constituents within the lamp are within the followingranges and satisfy the following criteria.

TABLE I Element: Partial pressure, atm. Sodium 1 10 -0.6 Thallium 0-10Cadmium 0-1.5 Mercury 03.0

Since thallium, cadmium, or mercury may be absent or present in a verysmall proportion, the following additional conditions must be met.

.(1) The sum of the partial pressures of mercury and cadmium are in therange of from approximately 0.05 atm. to 4 atm.

(2) Should the partial pressure of thallium be less than approximately1X10 atm., the pressure of cadmium is euqal to or greater thanapproximateliy 0.1 atm.

In the quaternary system, as is indicated hereinbefore, I prefer thatall four constituents be present, and it is preferred that they bepresent in the charge in such quantities as to cause the vapor inequilibrium with the nonvolatilized portions thereof have the followingpartial pressures:

TABLE 11 Element: Partial pressure, atm. Sodium 3 X 10- -03 Thallium 10-3 X 10 Cadmium 0.1-0.6 Mercury 0.05-1

Since all of the constituents are present in the ranges set forth inTable II and since the pressure of thallium does not fall below 10*atm., no further conditions need be added, except that the lampoperating temperature be such as to cause the partial pressures withinthe envelope to fall Within the above ranges and that the spectralemission have the peaks within the specified ranges as indicatedhereinbefore. Such a temperature range is approximately 750 C. to l,000C.

A further added requirement is placed upon the amount of sodium added tolamps in accord with the present invention. This is because sodium,being a reactive metal, tends to react with the high-density aluminacomprising the lamp envelope wall, in a preferred embodiment, to form asodium aluminate (NaAIO which causes thermal problems with the envelopewall and further reduces the light-transmissive characteristic thereof.In order to avoid the formation of sodium aluminate or the formation ofrelated complex aluminates from seal materials, the sodium partialpressure within the arc-tube must be maintained below specified valueswhich are a function of temperature. The basic chemical reaction forthis reaction is 3Na+2Al O +3NaAlO +Ai the maxi-mum tolerable sodiumpressure is governed by the relationship g1o Nn 5-61 X where P is inatmospheres and T is in degrees K. Some expletive maxima are as follows:

TABLE III Maximum partial Temperatures, C.: pressures of sodium, atm.680 0.05 780 0.30 900 0.90 1,000 2.40

This, together with the foregoing, constitutes the limits of constraintupon the addition of metallic materials to the charge of the lamps ofthe present invention.

In lamps of the invention, the materials other than sodium, which is anecessary constituent, which are chosen to comprise the charge fromwhich the light-emissive arc receives the arc-sustaining vapor, areadded for the following reasons.

Mercury is utilized because the sodium-mercury molecular complexradiates a valuable constituent in the red portion of the visiblespectrum. Additionally, in the presence of mercury, thecurrent-voltage-time starting characteristics are such as to facilitaterapid start of the main lightemitting arc. Mercury, being chemicallyinert, is useful in the lamp, in that the mercury atoms tend to bufferthermally the lamp envelope wall from the highly-corrosive sodium vaporsand makes it possible to substantially constrict the light-emitting arcto approximately two thirds to three quarters of the inner diameter ofthe arctube. Thus, although arc temperatures may raise to the order of4,000 C. and higher, it is readily possible to maintain the hottestportion of the envelope wall no hotter than 1,300" C. at the absoluteand 1,200 C. to l,250C. in a preferred embodiment. A very useful andunobvious result of the presence of mercury in a lamp envelopecontaining both sodium and thallium under the conditions of the presentinvention, wherein some of each constituent of the charge remainsnon-volatilized, is that the mercury uniquely controls and makespossible the optimization of the thermodynamics of the sodium-thalliumchemistry at ,the temperatures and pressures utilized in the lamps ofthe present invention.

Thallium and cadmium are found to be useful in the lamps of theinvention for the following reasons. Neither thallium or cadmiumcontribute in any way to the adverse attack of the vapors upon thealumina envelonpe wall. Similarly, they do not contribute to anydeleterious action upon the seals utilized to pass current-conductingelectrodes into the arc-tube. Additionally, the melting point andionization efficiencies of thallium and cadmium are within the rangessuch as to permit their use in a quaternary system and achieve thedesired chemical and luminescent output results. Finally, thallium andcadmium are useful in the lamps of the present invention in that noadditional getters are necessary, other than those normally provided ina high-pressure sodium vapor lamp, to remove deleterious constituentssuch as hydrogen, oxygen, or water vapor While the foregoing generalcriteria have been set forth as a framework within which theconstituents of the charge and operating temperatures of lamps in accordwith the present invention may be varied, more specific criteria arehereinafter set forth with regard to specific systems in specificvolumes and configurations of lamp envelopes. It will be appreciated,however, that for a given temperature of operation, partial pressuresand atom fractions of constituents in the excess are substantiallyindependent of lamp volume. As indicated hereinbefore, the lamps of thepresent invention, generally involve an outer envelope containing aninner envelope or arc-tube in which the arc-electrodes and metalliccharge are contained. Structural support for the inner envelope is alsocontained within the outer envelope, as may be a suitable gaseousatmosphere to help dissipate intense heat of the inner envelope andmaintain optimum operating conditions. The inner envelope is generallyconstructed of a high-density alumina ceramic or similarlight-transmissive polycrystalline ceramic having high resistance toactive metals and metal vapors such as sodium and sodium vapor atelevated temperatures of the order of l,200 C., or higher, and being atleast translucent and light transmissive to radiation within the visiblespectrum.

A 400 watt lamp, constructed in accord with the present invention,utilizes a high-density alumina polycrystalline cylindrical arc-tubehaving a 7 mm. inside diameter a length of approximately 11 cm., and aninterelectrode gap of approximately 8 cm.; and defining a volume ofapproximately 4 cubic centimeters. A quaternary system of sodium,thallium, mercury, and cadmium utilized at a lamp operating temperatureof approximately 780 C. may conveniently have the following ranges ofpartial pressure within the lamp envelope.

TABLE IV Element: Partial pressure, atm. Sodium l 10' 0.3 Thallium 0-4 X10 Cadmium 0-0.8 Mercury 04.0

Due to the possibilities of zero partial pressure of either of thallium,cadmium, or mercury, the following conditions must also be observed.

(1) Should the partial pressure of thallium be less than approximately 1l0- atm., the partial pressure of cadmium is at least approximately 0.1atm. As a corollary to this requirement, should the partial pressure ofcadmium be less than approximately 0.1 atm., the partial pressure ofthallium is at least as high as approximately 1x 10- atm.

(2) The sum of the partial pressures of cadmium and mercury is at leastapproximately 0.1 atm., but does not exceed approximately 4.0 atm.

Within the framework of the foregoing suitable ranges of partialpressures of the foregoing materials utilized at a lamp operatingtemperature of approximately 780 C., a preferred operating range is asfollows.

TABLE V Element: Partial pressures, atm. Sodium 3.2 10" -0.25 Thallium l3.0 Cadmium 0-0.6 Mercury 00.8

Since either cadmium or mercury may have zero partial pressure in theforegoing ranges, the following additional criterion must be met.

The sum of the partial pressures of mercury and cadmium is within therange of approximately 0.1 to 0.8 atm.

A specific example of one embodiment of a 400 watt lamp in accord withthe present invention falling Within the framework of the foregoinggeneralized criteria, contains the following partial pressures.

TABLE VI Element: Partial pressures, atm. Sodium 6 X 10* Thallium2.2)(10 Cadmium 0.23 Mercury 0.12

The foregoing partial pressures may be achieved within the lamp havingthe dimensions given above with the following proportions of theconstituents Within the charge.

A 400 watt, 4 cc. arc-tube lamp having this charge utilized a standardcommercial ballast such as that obtainable with Power Door, Cat. No.962490D3 from General Electric Company, Outdoor Lighting Department,Hendersonville, N.C., for high pressure sodium vapor lamps, andsustained an arc voltage of 101 volts, resulting in a lamp operatingtemperature of 780 C. The efficacy of this lamp was 110 lumens per wattwith near-white spectral emission.

As yet another set of governing criteria, the following data set forththe ranges of partial pressures in the broader aspects and also in thepreferred embodiments for a quaternary system involving sodium, cadmium,thallium, and mercury at a lamp operating temperature of approximately900 C. At this temperature, the broad suitable ranges of theconstituents and conditions, in terms of the partial pressures of theconstituents produced within the lamp envelope at the operatingtemperature, are

TABLE VIII Element: Partial pressure range, atm. Sodium 0.0010.6 Mercury03 Cadmium 0-1 Thallium 0-3 1() Since it is possible that any one of theabove-mentioned constituents in the group, mercury, cadmium, andthallium, can be absent, the following additional conditions apply.

(1) The sum of the partial pressures of cadmium and mercury is fromapproximately 0.05 to 3 atm.

(2) Should the thallium partial pressure be less than approximately 3X10 atm., the partial pressure of cadmium is greater than approximately0.1 atm.

At approximately 900 C., in a specific embodiment having superior, whitespectral emission, it is desired that the ranges of the addedconstituents be such as to provide the following partial pressures ofeach of the constituents.

TABLE IX Element: Partial pressure, atm. Sodium 0.0050.35 Cadmium 0.10.6 Mercury 0.05-l .0 Thallium 1 10 3 x 10- Within the generalframework of the aforementioned criteria, the following represents aspecific example of partial pressures, quantities of elements in theremaining reservoir, and the dosage thereof necessary to attain therequired partial pressure, at a lamp operating tempera- For purposes ofsimplicity, in accord with the present invention, I find that certainternary systems which are subspecies of the above-mentioned quaternarysystem, are suitable to provide lamps of high efiiciency and pleasingspectral response having white or sun-like spectral emission.

One such ternary system comprises the vapors of sodium, thallium, andmercury. One particular advantage of this system is that fewer metallicelements are required for the fabrication thereof.

For the range of operating temperatures of approximately 750 C. to 1,000O, the values of partial pressures and corresponding atom fractions ofthe constituents in the reservoir of each of the constituents of thisternary system are as follows.

TABLE XI Atom fraction in reservoir Partial ressure Element; at 750 C.at 1,000 O (atm.) p

Sodium 13-. 3 .02-. 52 1 x 10- to 06. Mercury. 015-. 25 015-.15 0.05 to4. Thallium... 58-. 33-. 98 1 x 10" to 1 x 10*.

For temperatures intermediate 750 C. and 1,000 C., the ranges ofpermissible atom fractions are intermediate those listed above. For anygiven charge the sum of the atom fractions is unity.

Preferably, however, the lamps of the present invention utilizing thisternary system are operated within the following range of partialpressures and atom fractions A specific example of a filling utilizingonly sodium, mercury, and thallium and producing high-efliciency,near-white light at a lamp operating temperature of approximately 900 C.is as follows.

TABLE XIII Atom fraction of Weight of Con- Partral pressure excesselement stituent added Element (atm.) in reservoir to charge (mg) Sodium0. 01 147 1. 0 Thallium. 0. 0022 835 50. 6 Mercury 0. 3 018 3. 0

In yet another embodiment of the present invention, the ternary systemof mercury, sodium, and cadmium may be utilized. This particular systemis advantageous in that, containing no thallium, the minimum operatingtemperature may be substantially lowered, as for example, toapproximately 650 C. while maintaining sufficiently high vapor pressuresof sodium, cadmium and mercury to achieve acceptable operatingcharacteristics. The general useful ranges of partial pressures ofconsituents of the ternary system herein is set forth in the followingtable for temperatures within the range of approximately 650 C. to 900C.

TABLE XIV Element: Partial pressure, atm. Sodium 0.01-0.2 Cadmium0.l-1.0 Mercury 0-4.0

As a special sub-category of the above system, is the sodium-cadmiumbinary which can also produce operative lamps. In this case the cadmiumpressure should lie in the range of 0.1 to 1.0 atm. The total pressureof mercury and cadmium must not exceed 4 atm. in any case.

Within the framework of the foregoing range of useful combinations, thefollowing table sets forth the preferred range of constituents for theternary system involving sodium, cadmium, and mercury, which range hasbeen found to produce high-efiicient, near-white-light-emitting,

metallic vapor arc-lamps.

TABLE XV Element: Partial pressure, atm. Sodium 0.015-005 Mercury 0l.00Cadmium 0.1-0.25

The foregoing ranges of pressures of the ternary system involvingmercury, cadmium, and sodium are useful at a lamp operating temperatureof as low as 680 C. to produce the desired result.

One specific embodiment of a lamp in accord with this embodiment of theinvention, involves a lamp with an operating temperature ofapproximately 680 C. having the same geometrical configuration anddimensions as set forth above, namely, a 7 millimeter ID wall and avolume of approximately 4 cubic centimeters, is constructed so as tohave the following partial pressures of each of the constituentsrelating to the partial pressures of each of the constituents relatingto the listed atom fraction of each of the constituents in excess of theliquid reservoir in the envelope and may be achieved by the addition ofthe listed weights of each of the constituents to the lamp envelope.

TABLE XVI From the foregoing, it is apparent that other ternary systemsincluding sodium and two other elements of the group including thallium,cadmium, and mercury may be devised and, with appropriate chargecompositions and lamp operating temperatures, utilized to emithighefficiency, chromatically-pleasing light.

Although thallium and cadmium have been selected as the ideal greencomponent source elements in accord with the present invention, and havebeen described in a quaternary system, it is possible to follow theteachings of the present invention to add certain other metallicvapor-radiating species, and to use more than four speciessimultaneously, providing that the thermodynamic criteria set forthherein are followed.

Other specie radiating in the green spectral range with strong lines aremagnesium, barium, strontium, copper, silver, scandium, lanthanium,cerium, praseodymium, and samarium. Thallium and cadmium, as greenemitters, have been chosen from the others above, as well as other greenemitters, for combination with mercury and sodium, under the conditionsand constraints set forth hereinbefore, as contributing to an idealsystem for the prodnotion of white-light-emitting vapor arc lamps havingthe ideal combination of good chromatic characteristics, highefliciency, and good maintainance. Their selection is due to theirunusual and superior compliance with the thermodynamic criteria setforth herein.

From the foregoing general criteria, detailed description and specificexamples of operative lamps in accord with the present invention, it isself-evident that I have provided a new and improved type of metal vaporarclamp containing sodium and other chemically compatible metal vaporspresent in thermodynamically consistent porportions, as to producesun-like, white light of very high efficacy, in excess of lumens perwatt.

The lamps of the invention are characterized by the identity and amountof the metals present in the vapor phase, by the lamp operatingtemperature, and by the all-important requirement that an excess of eachmetal in the charge be present in the liquid reservoir in equilibriumwith the vapor phase.

Thus, the lamps of the present invention may be characterized as excesslimited pressure lamps as contrasted to many prior art metallic vaporarc-lamps which may be characterized as amount limited pressure lamps.This difference has at least two distinct features which contribute tothe superiority of lamps of this invention over prior art amount limitpressure lamps.

In amount limited pressure lamps, any partial removal of any constituentfrom the vapor phase, as by clean-up or chemical activity, reduces thepartial pressure of that constituent, changes the thermodynamics of thelamp operation, and deleteriously afiects spectral emission andefficacy.

Additionally, it may be noted that the partial pressures utilized are,in many instances quite low. If an amount sufficient to yield only sucha partial pressure is added to a 400 watt or a 1,000 watt lamp, forexample, additions of less than one milligram of at least sodium must bemade. Obviously, in addition to the difficulty of such a task, accuratecontrol is extremely difficult to achieve.

The foregoing difiiculties are of great importance, for, as disclosedhereinbefore, if even one metallic charge constituent is amount limited,then so must be the others.

While the invention has been set forth hereinbefore with respect tocertain preferred embodiments and specific examples, many modificationsand changes will readily occur to those skilled in the art. Accordingly,by the appended claims, I intend to cover all such modifications andchanges as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric arc discharge lamp comprising:

(a) an hermetically-sealed, light transmissive ceramic envelope;

(b) a pair of solid arc-electrodes disposed in spaced relationship withone another and defining therebetween a discharge path for ahigh-current light-emissive are;

(c) a filling within said envelope of a starting gas having a relativelylow ionization potential;

(d) a thermally ionizable light-emissive charge within said envelope andincluding sodium and at least two of the materials selected from thegroup consisting of mercury, thallium, and cadmium,

(d said materials being present in concentrations such that a portion ofeach constituent of said 13 charge remains unvaporized at lamp operatingtemperatures,

(d such materials being present in said charge in quantities sufficientto provide at lamp operating temperatures of approximately 650 C. to1,000 C. the following partial pressures of said materials within saidenvelope,

(d sodiumfrom approximately 1X10- to less than 0.6 atm. (d mercury-fromto approximately 3 atm.

(d2c) thalliumfrom 0 to approximately atm.

(d cadmiumfrom 0 to approximately 1.5 atm.

(d said constituents being present within said envelop subject to thefollowing conditions,

((1 the sum of the partial pressures of cadmium and mercury must be fromapproximately 0.1 to 4 atm.,

(d should the partial pressure of thallium within the said envelope beless than approximately 1x10 atm., the partial pressure of cadmium mustbe at least approximately 0.1 atm.,

(d should the pressure of cadmium be less than approximately 0.1 atm.,the partial pressure of thallium must be greater than approximately 110- atm.;

(e) the total spectral emission of said lamp containing at least onedistinct component having a peak intensity within the wavelength rangefrom 5,050 A.U. to 5,500 A.U. and having a peak intensity of at least 10percent that of the peak intensity of said total spectral emission.

2. The lamp of claim 1 wherein said charge materials are present withinsaid arc-tube in quantities to produce therein at lamp operatingtemperatures of approximately 750 C. to 1,000 C. the following partialpressures:

(a) sodium-approximately 3X10 atm. to 0.3 atm.

(b) thalliumapproximately 1X10 atm. to 3X10 atm.

(c) cadmiumapproximately 0.1 atm. to 0.6 atm.

(d) mercury-approximately 0.05 atm. to 1.0 atm.

3. The lamp of claim 1 wherein said charge materials are present withinsaid arc-tube in quantities sufficient to produce therein at a lampoperating temperature of approximately 780 C. the following partialpressures:

(a) (a sodium-approximately l l0 atm. to 0.3

atm.

(a thallium -approximately 0 to 4X10" atm.

(a cadmiumapproximately 0 to 0.8 atm.

(a mercury-approximately 0 to 4.0 atm.

(b) said charges being present in said arc-tube subject to the followingconditions:

(b should the partial pressure of thallium be below approximately 1 10-atm., the partial pressure of cadmium is at least as high asapproximately 0.l atm.,

(b should the partial pressure of cadmium be below approximately 0.1atm., the partial pressure of thallium is at least as high asapproximately 1 l0 atm.,

(b the sum of the partial pressures of mercury and cadmium are withinthe range of approximately 0.1 atm. to 4.0 atm.

4. The lamp of claim 3 wherein the charge materials are present withinsaid arc-tube in quantities sufiicient to produce therein at a lampoperating temperature of approximately 780 C. the following partialpressures:

(a)(a sodiumapproximately 3.2 10* atm. to

0.25 atm.

(a thallium-approximately 1X 10- atm. to 3 10* atm.

(a cadmiumapproximately 0 to 0.6 atm.

(a mercuryapproximately 0 to 0.8 atm.

(b) said materials are present in quantities such as to satisfy thefollowing condition:

(b the sum of the partial pressures of mercury and cadmium is within therange of approximately 0.1 to 0. 8 atm.

5. The lamp of claim 4 wherein said charge materials are present withinsaid arc-tube in quantities to produce therein the following partialpressures at said lamp operating temperature:

(a) sodiumapproximately 6X 10- atm.

(b) thalliumapproximately 2.2 10- atm.

(c) cadmiumapproximately 0.23 atm.

(d) mercuryapproximately 0.12 atm.

6. The lamp of claim 1 wherein said charge materials are present withinsaid arc-tube in quantities sufficient to produce therein at a lampoperating temperature of approximately 900 C. the following partialpressures:

(a) (a sodiumapproximately 0.001 atm. to 0.6 atm.

*(a mercury-approximately 0 to 3 atm.

(a cadmiumapproximately 0 to 1 atm.

(a thalliumapproximately 0 to 3 10- atm.

(b) said materials being present in quantities suflicient to yieldpartial pressures which also satisfy the following conditions:

(b the sum of the partial pressures of cadmium and mercury is fromapproximately 0.05 to 3 atm.,

(b should the partial pressure of thallium be below approximately 3x10-atm., the partial pressure of cadmium is at least approximately 0.1atm.,

(b should the partial pressure of cadmium be less than approximately 0.1atm., the partial pressure of thallium is at least approximately 3 10-atm.

7. The lamp of claim 6 wherein said charge materials are present withinsaid arc-tube in quantities sufficient to produce therein at said lampoperating temperature the following partial pressures:

(a) sodium-approximately 0.005 atm. to 0.35 atm.

'(b) cadmiumapproximately 0.1 atm. to 0. 6 atm.

(c) mercury-approximately 0.05 atm. to 1.0 atm.

(d) thalliumapproximately 1 1=0- atm. to 3X10 atm.

8. The lamp of claim 7 wherein said charge materials are present withinsaid arc-tube in quantities sufiicient to produce therein at said lampoperating temperature the following partial pressures:

(at) sodium-approximately 0.3 atm.

(b) thalliurnapproximately 4.7)(10 atm.

(c) mercury-approximately 0.3 atm.

(d) cadmiumapproximately 0.12 atm.

'9. An electric arc discharge lamp comprising (a) anhermetically-sealed, light-transmissive ceramic envelope;

(b) a pair of solid arc-electrodes disposed in spaced relationship withone another and defining therebetween a discharge path for ahigh-current light-emissive arc;

(c) a filling within said envelope of a starting gas having a relativelylow ionization potential;

(d) a thermally ionizable light-emissive charge within said envelope andincluding sodium, mercury, and thallium,

(d said materials being present in concentrations such that a portion ofeach constituent of said charge remains unvaporized at lamp operatingtemperatures,

(d such materials being present in said charge in quantities sufficientto provide at lamp operating temperatures of approximately 750 C. to1,000 C. the following partial pressures of said materials within saidenvelope,

15 (d sodium-approximately 1 10 atm.

to 0.6 atm. (d mercury-approximately 0.05 atm. to

4 atm. (d thallium-approximately 1X10 atm.

to l atm.

'(e) the total spectral emission of said lamp containing at least onedistinct component having a peak intensity within the wavelength rangefrom 5,050 A.U. to 5,500 AU. and having a peak intensity of at least 10percent that of the peak intensity of said total spectral emission.

10. The lamp of claim 9 wherein said charge materials are present withinsaid arc-tube in quantities sufiicient at said lamp operatingtemperatures to provide therein the following partial pressures:

(a) sodium-approximately 2 l0- atm. to 0.3 atm.

(-b) mercury-approximately 0.08 atm. to 0.8 atm.

(c) thallium-approximately 2 10- atm. to

2.5 10- atm.

11. The lamp of claim 10 wherein said charge materials are presentwithin said arc-tube in quantities sufficient at a lamp operatingtemperature of approximately 900 C. to provide therein the followingpartial pressures:

(a) sodium-approximately 0.01 atm.

(b) thallium-approximately 0.0022 atm.

(c) mercury-approximately 0.3 atm.

12. An electric arc discharge lamp comprising:

(a) an hermetically-sealed, light-transmissive ceramic envelope;

(b) a pair of solid arc-electrodes disposed in spaced relationship withone another and defining therebetween a discharge path for ahigh-current lightemissive arc;

(c) a filling within aid envelope of a starting gas having a relativelylow ionization potential;

(d) a thermally ionizable light-emissive charge within said envelope andincluding sodium, cadmium, and mercury;

(d said materials being present in concentrations such that a portion ofeach constituent of said charge remains unvaporized at lamp operatingtemperatures,

(d such materials being present in said charge in quantities suflicientto provide at lamp operating temperatures the following partialpressures of said materials within said envelope,

(() mercury approximately 0 to 4.0 atm.

(e) said charge materials further being present in quantities sufficientto produce a sum of the partial pressures of cadmium and mercury ofapproximately 0.1 atm. to 4 atm.,

(f) the total spectral emission of said lamp containing at least onedistinct component having a peak intensity within the wavelentgh rangefrom 5,050 A.U. to 5,500 AU. and having a peak intensity of at least 10percent that of the peak intensity of said total spectral emission.

133. The lamp of claim 12 wherein the charge materials are presentwithin said arc-tube in quantities suflicient to provide therein at saidlamp operating temperatures the following partial pressures:

(a) sodiumapproximately 0.015 atm. to 0.05 atm.

(b) mercury-approximately 0 to 1.0 atm.

(c) cadmiumapproximately 0.1 atm. to 0.25 atm.

14. The lamp of claim 13 wherein said charge materials are presentwithin said arc-tube in quantities sufficient to produce therein at lampoperating temperatures of approximately 680 C. the following partialpressures:

(a) sodium-approximately 0.04 atm.

(b) cadmium-approximately 0.14 atm.

(c) mercury-approximately 0.10 atm.

References Cited UNITED STATES PATENTS 3,248,590 4/1966 Schmidt 313-1843,384,798 5/1968 Schmidt 313-184 3,453,477 7/1969 Hanneman et al 313-184JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US.Cl. X.R. 313-225, 229

